This invention relates to linear accelerators and, more particularly, to electron beam accelerators for use in free electron lasers. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Electron beam accelerators for use in electron beam collider devices or in free electron lasers (FEL) require electron source accelerators capable of delivering pulse trains of electron bunches of high charge density. A high electron density implies a high peak current (100 A to 2000 A) and a low normalized transverse beam emittance, for example, &lt;30 .pi..multidot.mm.multidot.mrad. A figure of merit for an electron beam is the "brightness" B.sub.n of the beam: EQU B.sub.n =2I/.epsilon..sub.x .epsilon..sub.y A/(m.sup.2 .multidot.rad.sup.2),
where I is the peak current and .epsilon..sub.x and .epsilon..sub.y are the normalized transverse emittances of the beam. A normalized emittance is defined to be EQU .epsilon..sub.n =.beta..gamma..epsilon.=4.pi..beta..gamma.[&lt;x.sup.2 &gt;&lt;x'.sup.2 &gt;-&lt;x.multidot.x'&gt;.sup.2 ].sup.1/2,
where the brackets &lt;&gt; denote an ensemble average over the electron beam, .gamma. is the relativistic factor, .beta. is the particle velocity divided by the speed of light, x is a transverse beam size, x' is a transverse beam divergence, and .epsilon. is the unnormalized emittance, where .epsilon.=.epsilon..sub.x =.epsilon..sub.y for an azimuthally symmetric beam.
One advance in electron accelerators producing an electron beam of high peak current, short burst duration, and high beam quality was the replacement of a conventional thermal electron emitter with an optically pulsed photocathode. The photocathode source is described in U.S. Pat. No. 4,715,038, "Optically Pulsed Electron Accelerator," issued Dec. 22, 1987.
It can be seen from the above discussion that the electron beam emittance must be maintained at a low value in order to increase the brightness of the beam at a given current. Many factors influence the beam emittance. For example, space charge effects adjacent the photocathode increase the emittance. Quadrupole fields along the accelerator are produced by conventional accelerating cavities and forces from the quadrupole fields tend to increase beam emittance.
However, new generations of FEL's are required for industrial, medical, and research applications, where the FEL is compact in size. Reducing the size of a FEL requires a concomitant reduction in size of the electron beam accelerator with a corresponding increase in accelerating field gradients and an aggravation of many factors that tend to increase beam emittance.
Accordingly, it is an object of the present invention to provide a compact linear accelerator for electrons and a high accelerating field gradient.
Another object of the present invention is maintain a low beam emittance at the high beam currents available from a photocathode electron source in order to maintain a high brightness beam.
One other object of the present invention is to eliminate quadrupole fields in at least the portion of the electron beam most affected by quadrupole fields.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.